Engraving, Marking and Coloring - All in one place.

Engraving or Marking, what is the difference?

What is the difference between marking and engraving metals from technology point of view? Marking is a process of changing optical properties of a metal surface without making changes on its profile. Engraving is a process of local removing some amount of material from a metal sample. Usually, engraving leaves a trace on a metal surface in the form of grove, whereas a trace of marking is color change on a marking surface.

Methods of Marking

There are three methods of metal marking by coloring. One of them is metal surface oxidation. In this method a thin layer of oxide is created on the metal surface with laser radiation. Laser radiation heats metal surface which comes into reaction with oxygen from the surrounding atmosphere.

An oxide layer creates a dispersive structure that selectively reflects incident light. Reflection characteristic of the surface depends on the optical properties of the layer material and its height h. Typically, color of the surface that an observer can see is determined by the oxide layer and optical properties of metal and an angle of light incidence, as well.

A second method of metal coloring is based on a dispersive characteristic of periodic structure created on metallic surface. This structure is made with high power pulse lasers. Pulse laser radiation creates plasmonic field on metallic surface and if radiation power density is high enough it creates periodic structure that diffracts incident white light. Formation of diffractive structures is made with lasers that emit pulses of light in the range of ns to ps and fluence 10-102 J/m2.

Visual effects an observer can see originates from splitting incident white light into a set of spatially separated multicolor beams. Colors of the beams depends strongly on an angle of incident of white light and an angle of observation.

Third method of coloring metallic surfaces - nanoparticles

In third method of metal coloring a pulse laser beam is used for creation nanoparticles on metallic surfaces. In this process, laser radiation heats metal surface and causes its melting and evaporation and then inducing a plasma plume. At the end of a laser pulse, during cooling a plasma plume nanoparticles are formed and deposited on a metal surface.

Color of metal surface depends on size and shape of nanoparticles. Dispersive characteristic of a surface could be controlled by changing parameters of laser radiation: peak power, fluence, pulse duration and wavelength. Usually, nanoparticles are created with lasers that emit short and very short pulses [ns – fs] and peak power density in the range of several 109 W/cm2

Laser Engraving

Metal engraving consists in removing small amount of material from metal surface. This way, a persistent trace is formed on a engraved material. Engraving could be made with a mechanic tool or with laser radiation. A high brightness laser beam could cause local temperature rising of a material and in the end leads to creating a groove on material surface. A process of creating grooves starts with melting a sample and then with rising temperature vaporization or even boiling proceeds.

The graph shows different regime areas of process of laser radiation with matter interaction. The lines for melting, spallation, and phase explosion correspond to relatively sharp thresholds for the onset of the corresponding processes, while the lines for vaporization and plasma formation are defined at a qualitative level and correspond to the conditions when vaporization makes a significant contribution to the material ejection and a substantial degree of ionization is reached in the ablation plume, respectively.

Main factors influencing the process of laser engraving are radiant exposure on material surface and laser pulse duration. The longer is a laser pulse the higher radiant exposure is needed to obtain vaporization of a material. Pulse duration influences processes of creating shock waves in a processed materials. Very short laser pulses, that in [fs –ns] range, could create not only evaporation of a material but can lead to phase explosion (ablation) and plasma formation. These processes could be hardly achieved with cw lasers.

How to Choose an Appropriate Laser Engraver

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The main factor between material types that decides if a particular material type can be easily laser engraved or not is whether the material is metallic or not. Some metallic materials cannot be engraved at all. To engrave metals, ie. to remove a layer of the processed metal, it is recommended to start with a 15 W or higher optical power blue laser head. As of now, PLH3D-15W laser engraver is the best compact laser head on the market capable of engraving steel as well as stainless, carbon, tool and high-speed subspecies of it. It still is not powerful enough to engrave on bare Aluminium, Copper or brass simply because these materials either conduct heat too well or reflect too much laser light. It is possible to laser engrave copper with 30 W (optical power) blue laser heads. XT-50 is another laser engraver that allows you to work with metals, and it does so with outstanding precision (as its 50 µm wide focused beam spot corresponds to >500 real-life DPI ). These two engraving laser heads allow you to laser engrave (not just mark) the metals mentioned above.

If you are satisfied with simply laser marking the metal, you can achieve it on stainless steel and Titanium (but not other metals) with 6 W laser engravers. Laser marking is defined here as changing the color of the metal surface as a result of overheating. Laser marking is however less durable than laser engraving because the depth of the color change is much lower. Laser-marked surfaces can withstand just a few scrubs with fine sandpaper. For this purpose, the XF+ laser engraver with High-Resolution Lenses.

Nevertheless, if you want to do it really fast (>30 mm/s), then you will either need a laser with 15 W of optical power or the XT-50 laser engraver and cutter. Depending on the chemical composition and type of stainless steel you can assume speeds of 0.5 - 2 mm/s, 8 -15 mm/s, or 20-40 mm/s for the XF+ High Resolution Lens,PLH3D-15W respectively.

Blue lasers for coloring and engraving metals

Time-energetic zones of main processes during laser processing of metals are shown in the next graph. It is hard to reach explosion zone conditions with the radiant exposure below 109J/cm2s. This corresponds to 100W laser power in quasi pulse mode with pulse duration 10µs focused into a spot of area 0.1mm by 0.1mm. So, metal processing has to operate in the boiling/vaporization zone shown in the graph.

Engraving as well as coloring metals can be done with laser radiant exposure on a level in the boiling/vaporization zone. Apart from radiant exposure, there are another important parameters of metal surface processing with blue lasers. Blue lasers usually operate in the quasi-CW mode with pulse duration of several µs. They are mounted in scanning systems such as CNC machines or galvo scanners.

For such systems pulse duration and scanning rate have to be chosen to achieve best condition of coloring/engraving for a particular processed material type.

Examples of laser processing of metals